In the manufacture of extruded foam products such as styrene sheet, board or billets, the extruded products are subjected to cutting and trimming, and defect or off grade rejections, particularly on start-up of the continuous extrusion line. The operations produce a substantial amount of scrap, which can range from large pieces to sawdust. The scrap is economically recycled.
Large pieces of scrap are run through a scrap chopper, collected in a storage silo, and then the smaller particulate material is run through a reclaim extruder. The material is remelted and the melt extrudate is chopped and cooled to form feed pellets which are then added to the virgin raw material supply of the continuous extrusion line.
The chopped reclaim material from the scrap chopper or silo has the consistency and low density of fluff. The material is so light that it is often conveyed from the silo to the reclaim extruder with forced air conveyors or grain conveying augers.
The reclaim extrusion rate or output is a direct function of the mass of the material entering the feed throat of the reclaim extruder. Thus the lighter the density of the material, the lower the through-put or extrusion rate. The through-put or rate is normally measured in pounds per hour.
Many attempts have been made to improve the through-put of reclaim extruders. These usually involve the use of a force-feed section in the throat of the reclaim extruder. These force-feed sections generally take the form of one or more augers, supposedly designed to push or compress the light fluffy material into the feed section of the reclaim extruder. One prior attempt involved three rotating augers driven by a DC motor and drive chains. The three auger device was extremely complex and a nightmare to maintain. The rate, when operating properly, only increased about 50 to 200 pounds per hour for a total rate of about 500-700 pounds per hour, depending on the density of the fluff material entering the extruder. One of the problems was overpacking the feed throat, and this would lead to "rat holing," where the auger simply forms a hole in the overpacked material. This would then cause loss of feed to the reclaim extruder. It has been characterized as pushing marshmallows with a bulldozer, since the system had no feel or ability to react to the light density material.
On the other hand, when heavier density material was introduced, the system could quickly become overloaded and stall, also shutting down the system completely. Accordingly, with rat holing, frequent stalling, complexity in structure, difficulty in maintenance, and, more importantly, minimal or no improvement in through-put, the three auger system was abandoned as unworkable.
Accordingly, it would be desirable to have a system which would be more rugged in design, easier to maintain and operate on a continuous and reliable basis, but, more importantly, one which would substantially increase the output rate of the reclaim extruder.